In the design of precision circuits, the immunity of a circuit to the effects of non-linearity, noise, and common mode voltages tends to be a priority. In slower applications that require low power, these concerns can be alleviated easily. In many applications however, requirements for high speed and high precision are necessary and achieving this while maintaining low power is not readily obvious. This makes the implementation of devices such as a simple source follower more complex especially as one attempts to achieve a low power output.
Two types of common follower devices are source followers and emitter followers. A source follower is a configuration of a field-effect transistor (“FET”), whereupon an input signal is sent to the gate terminal of the FET, and the output is taken at the source terminal. In such a configuration, the drain of the FET is common to both the input and output. A load, such as a resistor or capacitor, is often connected to the source at the output. An emitter follower uses the same configuration, but instead of using a FET, a bipolar junction transistor (“BJT”) is used, whereupon the input signal is sent to the base terminal of the BJT and the output is taken at the emitter terminal.
One of the predominant problems with follower devices is that if the input to the follower changes suddenly and the output is capacitively loaded, the output is unable to settle quickly. Furthermore, if the output is disturbed, it cannot settle back quickly if a large capacitor is connected as a load.
At low power levels, settling the output voltage is difficult because the follower's standing current is of low amperage and is insufficient to settle the output quickly. This may be addressed by attempting to increase the standing current itself, but doing so would result in a power increase to the circuit. Thus designing a circuit to increase the slew rate without increasing the power consumption or noise is essential.
Prior enhanced slew rate follower devices have included the use of additional transistors to achieve biasing of the follower, by stacking the transistors on top of the device. The limitation that these configurations have is that they implement additional devices that are always in an on-state. Though such proposed solutions might seemingly be suitable for low power applications, the presence of a continuously-on device ultimately leads to additional circuit noise. These configurations also compromise the common mode input voltage range because of the presence of additionally inserted circuit elements.
The present invention presents a configuration which allows for the enhancement of the slew rate while maintaining a lower power and high precision output. Furthermore, the present invention overcomes the problem of the additional noise associated with previous configurations, without compromising the input voltage range.